The present invention relates to impact tools in general, and more particularly to improvements in pneumatic impact tools of the type wherein a hammer is acted upon by a compressed gaseous fluid to strike against a tool or against a tool holder and to thereby cause penetration of the working end of the tool into a selected material. As a rule, the hammer is or includes a piston or plunger which is reciprocable in the chamber of a cylinder and can be acted upon by compressed gaseous fluid (e.g., air) which is admitted against its rear surface, i.e., against that surface which faces away from the tool.
When a prosthesis (e.g., an artificial knee or hip joint) is to be implanted in a living organism, it is often necessary to gain access to the cavity which confines the marrow in a bone and to configurate the cavity with a view to provide therein room for reception of the anchoring portion of the prosthesis. In many instances, the implantation is completed by filling the cavity around the inserted anchoring portion of the prosthesis with a bone cement which adheres to the bone as well as to the prosthesis and completely fills the cavity around the inserted anchoring portion. It is also known to impart to the anchoring portion of a prosthesis the shape of a dull wedge with a relatively small angle between its mutually inclined surfaces and to treat the surfaces surrounding the cavity in a bone for the purpose of ensuring a desirable tight fit of the wedge-like anchoring portion therein. This obviates the need for bone cement because the anchoring portion fits snugly into the cavity and is in pronounced surface-to-surface contact with the surrounding material of the bone. The treatment of the surfaces surrounding the cavity in the bone is effected by means of suitable rasping, grating or scraping tools which are supplied in different widths. The specialist in charge of enlarging the cavity begins with the narrowest tool and thereupon resorts to tools having greater widths until the dimensions of the cavity match the optimum dimensions. The comparison between the actual dimensions and the desired or optimum dimensions of the cavity is made by taking X-ray pictures. This ensures that the anchoring portion of the prosthesis and/or any other portion which must be inserted into the bone is a tight fit therein and is not likely to become loose after the implantation of the prosthesis is completed. The surfaces bounding the cavity must be formed by removing the hard tissue (compacta) of the bone subsequent to removal of the softer spongiosa.
In order to drive the scraping tool into the bone, the specialist in charge of such operation normally employs a so-called slide hammer which is manipulated by hand. Such hammer comprises an elongated rod one end of which carries the socket for the scraping tool. The rod further supports a slidable weight or mass which is propelled by hand against an anvil forming part of or connected to the socket for the scraping tool.
The just described mode of enlarging the cavity in a bone is cumbersome, time-consuming and tiresome to the person doing the job. Moreover, the instrument invariably performs at least some wobbling movement as a result of manual propulsion of the weight against the anvil in order to ensure that the scraping tool can be more or less readily retracted or extracted from the cavity. While such wobbling is desirable on the ground that it reduces the likelihood of jamming of the scraping tool in the cavity, it evidently affects the accuracy of the shaping operation. In addition, the rate at which the tool removes material of the compacta is rather low so that several hundred impacts are required before the operation is completed. It will be readily appreciated that such work (e.g., the enlargement of the cavity in a femur) is extremely tiresome and the quality of work must deteriorate as the removal of material from the bone progresses.
It is also known to employ impact type chisels and/or hammers which are used in conjunction with the aforementioned scraping tools to drive the latter into the material of the bone. Since the use of impact type tools reduces the likelihood of wobbling, the chisel is likely to be stuck in the bone so that its removal necessitates the exertion of a substantial effort and involves the danger of breaking the bone and/or removing excessive quantities of bone material during extraction of the scraping tool. Therefore, such impact type tools failed to replace the aforementioned slide hammers.